The natural hormone, 1α,25-dihydroxyvitamin D3 (also referred to as 1α,25-dihydroxycholecalciferol and calcitriol) and its analog in the ergosterol series, i.e. 1α,25-dihydroxyvitamin D2 are known to be highly potent regulators of calcium homeostasis in animals and humans, and their activity in cellular differentiation has also been established, Ostrem et al., Proc. Natl. Acad. Sci. USA, 84, 2610 (1987). Many structural analogs of these metabolites have been prepared and tested, including 1α-hydroxyvitamin D3, 1α-hydroxyvitamin D2, various side chain homologated vitamins, and fluorinated analogs. Some of these compounds exhibit an interesting separation of activities in cell differentiation and calcium regulation. This difference in activity is useful in the treatment of a variety of diseases as established in the art, such as renal osteodystrophy, vitamin D-resistant rickets, osteoporosis, psoriasis, and certain malignancies (see for example, Zemplar, Calcipotriol, MC-903, Dovonex, 22-oxa-1α,25-(OH)2D3) Slatopolsky, E., Finch, J., Ritter, C., Denda, M., Morrissey, J., Brown, A. & DeLuca, H. (1995) Am. J. Kidney Dis. 26, 852-860; Kubodera, N., Sato, K. & Nishii, Y. (1997) in Vitamin D, eds. Feldman, D., Glorieux, F. H. & Pike, J. W. (Academic, New York), Vol. 63, pp. 1071-1086; Calverley, M. J. (1987) Tetrahedron Lett. 43, 4609-4619; Uskokovic, M. R., Studzinski, G. P. & Reddy, S. G. (1997) in Vitamin D, eds. Feldman, D., Glorieux, F. H. & Pike, J. W. (Academic, New York), Vol. 62, pp. 1045-1070; Kensler, T. W., Dolan, P. M., Gange, S. J., Lee, J.-K., Wang, Q. & Posner, G. H. (2000) Carcinogenesis 21, 1341-1345; Binderup, L., Binderup, E. & Godfredsen, W. O. (1997) in Vitamin D, eds. Feldman, D., Glorieux, F. H. & Pike, J. W. (Academic, New York), Vol. 61, pp. 1027-1043; Jones, G. (1997) in Vitamin D, eds. Feldman, D., Glorieux, F. H. & Pike, J. W. (Academic, New York), Vol. 58, pp. 973-994; Brown, A. J. & Slatopolsky, E. (1997) in Vitamin D, eds. Feldman, D., Glorieux, F. H. & Pike, J. W. (Academic, New York), Vol. 59, pp. 995-1009; Shankar, V. N., Propp, A. E., Schroeder, N. S., Surber, B. W., Makin, H. L. J. & Jones, G. (2001) Arch. Biochem. Biophys. 387, 297-306. All these references are incorporated herein by reference for all purposes.
As discussed above, renal osteodystrophy is a bone disease that occurs when the kidneys fail to maintain the proper levels of calcium and phosphorus in the blood. Renal osteodystrophy is a common problem in people with kidney disease and affects 90 percent of dialysis patients.
Renal osteodystrophy is most serious in children because their bones are still growing. The condition slows bone growth and causes deformities. One such deformity occurs when the legs bend inward toward each other or outward away from each other; this deformity is referred to as “renal rickets.” Another important consequence is short stature. Symptoms can be seen in growing children with renal disease even before they start dialysis.
The bone changes from renal osteodystrophy can begin many years before symptoms appear in adults with kidney disease. The symptoms of renal osteodystrophy are not usually seen in adults until they have been on dialysis for several years. Older patients and women who have gone through menopause are at greater risk for this disease because they're already vulnerable to osteoporosis, even without kidney disease. If left untreated, the bones gradually become thin and weak, and a person with renal osteodystrophy begins to experience bone and joint pain and an increased risk of bone fractures.
In healthy adults, bone tissue is continually being remodeled and rebuilt. The kidneys play an important role in maintaining healthy bone mass and structure because it balances calcium and phosphorus levels in the blood. If calcium levels in the blood become too low, the parathyroid glands release parathyroid hormone (PTH). This hormone draws calcium from the bones to raise blood calcium levels. Too much PTH in the blood causes disturbances in calcium and phosphorus homeostasis. This in turn removes too much calcium from the bones; over time, the constant removal of calcium weakens the bones.
In this context, secondary hyperparathyroidism is characterized by an elevation of PTH associated with inadequate levels of active vitamin D hormone. Typically, Vitamin D requires two sequential hydroxylations in the liver and the kidney to bind to activate the Vitamin D receptor (VDR). The endogenous VDR activator, calcitriol [1,25(OH)2 D3] is a hormone that binds to VDRs that are present in the parathyroid gland, intestine, kidney, and bone to maintain parathyroid function and calcium and phosphorus homeostasis, and to VDRs found in many other tissues, including prostate, endothelium and immune cells. Phosphorus also helps regulate calcium levels in the bones. Healthy kidneys remove excess phosphorus from the blood. When the kidneys stop working normally, phosphorus levels in the blood can become too high, leading to lower levels of calcium in the blood and resulting in the loss of calcium from the bones.
Healthy kidneys produce calcitriol to help the body absorb dietary calcium into the blood and the bones. If calcitriol levels drop too low, PTH levels increase, and calcium is removed from the bones. Calcitriol and PTH work together to keep calcium balance normal and bones healthy. In a patient with kidney failure, the kidneys stop making calcitriol, dietary calcium is not absorbed and calcium is removed from the bones.
Controlling PTH levels prevents calcium from being withdrawn from the bones. Usually, overactive parathyroid glands are controllable with a change in diet, dialysis treatment, or medication. The drug cinacalcet hydrochloride (Sensipar), approved by the Food and Drug Administration in 2004, lowers PTH levels by binding to the calcium receptor that controls PTH release. If PTH levels cannot be controlled, the parathyroid glands may need to be removed surgically. Other treatments for the condition include taking synthetic calcitriol as a pill or in an injectable form.
Renal osteodystrophy can also be treated with changes in diet. Reducing dietary intake of phosphorus is one of the most important steps in preventing bone disease. Often, medications such as calcium carbonate (Tums), calcium acetate (PhosLo), sevelamer hydrochloride (Renagel), or lanthanum carbonate (Fosrenol) are prescribed with meals and snacks to bind phosphorus in the bowel, which decreases the absorption of phosphorus into the blood.
Other treatment choices for renal osteodystrophy include Paricalcitol, the active ingredient of Zemplar (paracalcitol injection, USP), which is a synthetic, biologically active vitamin D analog of calcitriol with modifications to the side chain and the A (19-nor) ring. Preclinical and in vitro studies have demonstrated that paricalcitol's actions are mediated through binding to the VDR, resulting in the selective activation of Vitamin D response pathways. Calcitriol and paricalcitol have been shown to reduce parathyroid hormone levels by inhibiting PTH synthesis and secretion.

The structure of 1α,25-dihydroxyvitamin D3 and the numbering system used to denote the carbon atoms in this compound are shown below.
